Various forms of solid state sensor arrays such as Charge Couple Devices (CCDs), and Complimentary Metal Oxide Semiconductor (CMOS) sensors are currently in use in electronic image capture systems. These solid-state sensors have an imaging surface with an array of photosensors. Light from a scene is focused onto the array of photosensors. Each photosensor generates a signal representative of the number of photons striking the photosensor within an exposure period known as an integration time. The signals from the photosensors are extracted by an image processor and contrast differences between signals from the photosensors are used to form an image of the scene.
The photosensors on a solid-state imager are capable of generating a signal that are representative of the amount of light striking the photosensor when the light striking the photosensor during the integration time is within a predefined range of illumination intensities. This range of illumination intensities is known as the dynamic range of the solid-state imager. The dynamic range of the solid-state imager is bracketed by a lower response threshold and an upper response threshold. The lower response threshold is set by the exposure level at which the inherent signal to noise properties of the sensor material and the electronic circuitry designed to extract stored information from the sensor approaches the inherent Poisson signal to noise ratio of the exposing light. The upper response threshold is set by the inherent charge-storage capacity of the photosensitive substrate, typically doped silicon. An introductory description of this problem appears in “Silver Halide and Silicon as Consumer Imagers,” by R. P. Szajewski, J Image Sci. & Tech, Vol. 45, page 326 (2001).
The human eye is also has photoreceptors that are capable of detecting light that is within a range of illumination intensities. However, the dynamic range of solid-state imagers is less than the dynamic range of the human eye. Thus, what is needed is a solid-state imager and imaging system having a wider dynamic range.
One reason that solid state sensors lack the dynamic range of the photoreceptors of the human eye is that a large portion of the imaging surface of most solid state imagers is dedicated to regimes intended for interconnecting the individual photosensor areas, to regimes intended to act as drains for excessive charge and to insulation patterns electrically isolating the individual photosensors. Accordingly, a large proportion of light that is focused on the imaging surface strikes portions of the imaging surface that are not photosensitive. This wastes the energy from such light and causes such solid state sensors tend to have reduced ability to capture images of scenes having relatively low levels of illumination.
An approach for improving the sensitivity of solid state sensors is described in U.S. Pat. No. 4,667,092 entitled “Solid-state image device with resin lens and resin contact layer” filed by Ishihara on Dec. 22, 1993. In the '092 patent, individual micro-lenses are associated with individual photosensors on an imager. A distinct camera taking lens focuses light from a scene at a focal plane. The solid-state sensor and micro-lens are positioned at the focal plane. Each micro-lens collects light falling onto portions of the imager near a photosensor with which the micro-lens is associated and focuses the light onto the associated photosensor. This increases the useful sensitivity of the array and shifts the lower response threshold of the sensor so that the sensor array can capture images of scenes that have lower levels of illumination. The sensitivity enhancing micro-lenses of the '092 patent have been used to enhance the sensitivity of the CCD and CMOS sensors.
However, the upper response threshold experiences an equivalent shift which reduces the ability of the sensor array to capture differentiable images under higher illumination conditions. Thus, the sensitivity enhancing micro-lenses of the '092 patent shift but do not extend the dynamic range of the sensor.
Another approach for improving the dynamic range of solid state sensors is to adjust the integration time to compensate for different levels of scene illumination. In low illumination scenes, the integration time is extended, which permits more charge to accumulate in each photosensor and effectively shifts the dynamic range of the photosensor to a lower level. In higher illumination scenes, the integration time is reduced. This limits the amount of charge that accumulates at each photosensor to prevent over exposure. This approach maximizes the utility of the available dynamic range of the photosensor. However, this approach does not provide a solution for capturing electronic images of scenes in which there is a range of illumination conditions that are within the dynamic range of human vision but outside the dynamic range of the image sensor.
Yet another approach to improving the sensitivity of a solid state imager entails electronically “ganging” proximate photosensitive sites to increase the effective photon capture associated with the ganged sites while maintaining the charge storage to capacity of the ganged sites. This approach adventitiously provides for a limited increase in sensor dynamic range since the lower response threshold is dropped while the upper response cutoff is maintained. However, sensor resolution is greatly reduced.
Other attempts to improve the limited dynamic range of solid-state sensor arrays have also been made. One approach is to use an image sensor having a non-linear response to light from a photographic scene. For example, Fill Factory NV of Mechelen, Belgium has developed a solid-state sensor known as the “FUGA 1000” which has a logarithmic response to light from a scene. The non-linear response of the sensor permits image information to be obtained over an extended dynamic range. Such sensors have many useful applications. However, in certain circumstances a solid state sensor having a non-logorithm linear response is preferred.
In related art, U.S. Pat. No. 5,471,515 entitled “Active Pixel Sensor with Intra Pixel Charge Transfer” filed by filed by Fossum et al. on Jan. 28, 1994 and U.S. Pat. No. 5,841,126 entitled “CMOS Active Pixel Sensor Type Image System on A Chip” filed by Fossum et al. on Jan. 24, 1997 describe the integration of light sensitive structures and computational structures on a single complementary metal oxide semiconductor (CMOS) substrate to form a “camera-on-a-chip.” These integrated devices potentially allow multiple image capture and digitization cycles per scene exposure. The images captured during these cycles can potentially be combined in digital space to create a single image with an apparent dynamic range that is greater than the actual dynamic range of the sensor. However, the CMOS sensors described in the '515 and '126 patents for the purposes described are noted for their poorer inherent dynamic range and their inherent high signal to noise ratio. Further, the process of capturing multiple sequential images can create problems where, for example, the content of the scene rapidly changes, thus making the approach suitable for static but not dynamic scenes requiring a fast shutter.
Meyers, in U.S. Pat. Nos. 5,676,371, 5,751,492, 5,731,899, 6,137,535, 6,141,048, 5,796,522, 5,822,125 and 5,812,322 describes the use of arrays of multiple small lenses as replacements for primary camera lenses in combination with known solid state sensors to enable highly compact cameras. Here, multiple sensor active sites are associated with each small-lens and the shortened focal length of the small lens enables the construction of highly miniaturized cameras by omission of the primary camera lens and associated optical train. The use of micro-lenses in these patents however, does not improve the dynamic range of the camera system, because each of the multiple lenses is employed as a conventional primary camera lens.
U.S. Pat. No. 6,381,072 entitled “Lenslet Array System and Method, filed by Berger et al. on Jul. 20, 2000, describes a structure useful in adapting a conventional film camera system for use as a digital image capture system. The camera system of the '072 patent employs multiple refractive and diffractive lenses to form a stacked array magnifiers (SAM) suitable for adapting a film camera for use with a smaller area solid-state sensor. The SAM is used because film cameras are typically adapted to focus light onto a segment of a photographic film that has an imaging surface are that is substantially larger that the imaging surface area of most solid state imagers. Accordingly, the SAM adjusts the size of the image formed by the optical system of the film camera so that the film camera forms an image at the solid state imager that conforms to the size of the electronic imager. However, the use of the SAM as described in the '072 patent does not enhance the dynamic range of the imager.
Thus, the problem of the limited dynamic range in solid state sensors and image capture systems that use such sensors has yet to be overcome and what is needed is a solid state imaging system having improved effective dynamic range.